Someone who appears to be innocent when it comes to sex, but what lies under this person's innocent appearance is a sexual freak. Could apply to "tame" looking people who are sexually wild, or a person who is into sexual kinks. Can apply to women and men, but used often to refer to women.

I brought this one girl home. I was afraid of what to do because she seemed really uninterested in being more than friends, but when I walked in on her completely naked in my room, I realised I had brought home an undercover freak.

The girl who claims that she is not sexually active, and never has been, but at a closer look has a different guy over every weekend to "watch a movie." She will claim that they just "snuggle," but it becomes harder to believe with each stranger she brings home.
It is possible she is a compulsive liar, or it is possible that she just has paternal "daddy" issues.

Guy A: "Yo, that girl Haley isn't down for anything."
Guy B: "Nah, bro, word on the street is she's an Undercover Freak, and she's good. She just keeps it on the down low."